In an aspect, the present invention generally relates to producing a high freeness pulp from lignocellulosic feed material via mechanical and/or chemi-mechanical refining techniques that may be suitable for paperboard and absorbency grade applications.
Techniques for mechanical and chemi-mechanical refining are known. For example, U.S. Pat. No. 7,758,720 describes a mechanical or chemi-mechanical refining system that includes three major subsystems: preprocessing, pretreatment, and production or primary refining.
The '720 patent describes a conventional preprocessing subsystem, such that a feed material comprising wood chips is washed then maintained in a pre-streaming bin or the like at atmospheric conditions for a period of time typically in the range of 10 minutes to 1 hour before being conveyed to the pretreatment subsystem.
The '720 patent further describes a pretreatment subsystem that includes a pressurized rotary valve for maintaining pressure separation between the preprocessing subsystem and the balance of the pretreatment subsystem, a pressurized compression device (such as a screw press), a decompression zone or decompression region (which may be part of the screw press or connected to the discharge of the screw press), and a fiberizing device (such as a disc or conical refiner).
As described in the '720 patent, the environment within the compression device, the decompression zone, and the fiberizer are all maintained at a saturated steam atmosphere in the range of about 5-30 psig (i.e., 0.3-2.1 bar). The '720 patent describes a transfer screw interposed between the pressurized rotary valve and the compression device, whereby the time period during which the chips in the transfer screw are exposed to the saturated steam pressure and temperature conditions, before entering the screw press, can be controlled. As a minimum, the '720 patent teaches that chips should be conditioned for a period of 5 seconds in a saturated steam atmosphere at 5 psig pressure.
The principle application for pulps produced using the method of the '720 patent were for mechanical printing paper applications including newsprint and magazine grades; such pulps are typically produced at lower freeness, less than 150 ml, and require significant energy application. An important goal of the '720 patent was to lower the energy consumption of energy intensive mechanical printing grade pulps.
It is understood in the art that high preheating and refining temperatures, well above the lignin softening temperature, are preferred in order to produce high freeness mechanical pulp from wood chips with low shive content. Such pulps are well suited for paperboard middle ply applications and are preferably refined at pressure levels in the range of 90 psig or higher; well above that used to produce mechanical pulps for printing paper applications. A drawback of these higher pressure pulps is lower pulp brightness. A chemi-thermomechanical pulping technique utilizing such high temperatures for high freeness pulp is disclosed, e.g., in U.S. Pat. No. 5,879,510.
It is believed that the prior art for producing higher freeness mechanical pulps, while achieving acceptable properties such as low shive content and good inter-ply strength, may be deficient in other aspects including long fiber content (too high), long fiber bonding (too low), inter-ply surface strength at a given bulk (too low), unbleached and bleached pulp brightness (too low). Incremental gains in surface strength without a sacrifice (reduction) in pulp bulk is an extremely important attribute for enhancing the competitiveness of paperboard middle layer pulps.